Next SQL, which verifies if customer row exists in database:

SELECT customer0_.CUSTOMER_ID AS x0_0_
FROM   T_CUSTOMER customer0_
WHERE  (customer0_.CUSTOMER_ID = 123456)

Will be generated with next NHibernate Query:

public bool CustomerExistsInDatabase(Int32 customerId)
{
    var result = Session.CreateQuery(string.Format(“select c.CustomerID from Customer c where c.CustomerID = {0}”, customerId)).UniqueResult<Int32?>();

    return result.HasValue;
}

Please note, that you could use not only primary key, but any other property in your where condition.